This study developed an analytical method to quantitatively analyze the dual-mode seeker guidance performance. The method derives the seekers' measurement models and implements the Kalman filter. The study takes the RF/IR dual-mode seeker, which is proposed for advanced long/medium range air-to-air missiles, as the example. The two-dimensional guidance loop is used to illustrate the methodology and dimensionless equations are derived for generality. After discussing several missile-target scenarios, the results show that the developed scheme provides sufficiently accurate target information for missile guidance, not only for dual-mode seekers, but also when one of the seekers is jammed. The methodology developed in the study can also be applied to increase the accuracy of target tracking.